In digital copying machines and facsimile devices, an image processing device for performing image processing against image signals gained by reading a document comprising a letter portion, a photograph portion or a dot portion, or the mixture of these plurality of portions by CCD image sensors and the like to improve the image quality of the recording image. This image processing is performed by identifying whether the read image type is either a letter image, a photograph image, or a dot image, and according to the identification result, performing the best processing to each pixel.
For example, Japanese Laid-Open Patent Publication No. 6-150059 discloses an invention where document image is divided into blocks comprising a plurality of pixels, and the image type of the document image is identified by matching patterns. In this invention, a plurality of basic patterns are prepared, and by comparing the document image pattern with the basic patterns, the image type of the document image is decided.
Further, in Japanese Laid-Open Patent Publication No. 8-149297, an invention for identifying whether an observation pixel is included in a dot image portion or not by utilizing the periodicity of the image data of dot images and detecting the periodicity of the image data of a line in the document image is disclosed.
Still further, Japanese Laid-Open Patent Publication No. 61-194968 discloses a dot photograph region identification method wherein a pixel inside a block having different signal levels in the main scan direction and the sub scan direction is measured, and by comparing the sum of the change coefficient, identifying whether the observation pixel is of a dot photograph image portion or not.
However, in the image identifying process by matching patterns as disclosed in Japanese Laid-Open Patent Publication No. 6-150059, it is necessary to prepare many patterns beforehand, which leads to the need of a large memory capacity, and less universality.
Further, in the method of detecting the periodicity of the image data of one line as disclosed in Japanese Laid-Open Patent Publication No. 8-149297, there are cases where the dot portion could not be identified reliably, since in the case of a dot region with low density and small dot sizes, dots are rarely placed on one line. Therefore, it is necessary to detect periodicity of the image by plurality of lines in two dimension regions, which leads to necessity of many block memories according to the periodicity of the dots, which further leads to rise of costs.
Even further, according to the invention where continuity of dot photograph portions is detected as is disclosed in Japanese Laid-Open Patent Publication No. 61-194968, the detection of dot regions may be possible, but the identification between the photograph portion having small difference in signal levels with the letter portion is difficult, and accurate identification of dot photograph portions could not be realized.
Therefore, the object of the present invention is to provide an image processing device enabling a reliable identification of whether which pixel of the document image is included in which of the plurality of image regions by a small circuit structure.